Retrieve a random row in a user defined function?

Question

I'm trying to define this function:

CREATE FUNCTION getRandomName  ()
    RETURNS VARCHAR(48)
AS BEGIN
    -- concatenate two random strings from two columns in a table and return as a new string
    DECLARE @finalStr VARCHAR(48);
    SET @finalStr = (SELECT TOP 1 st1 FROM randomStrings ORDER BY RAND()) + 
        ' ' + 
        (SELECT TOP 1 st2 FROM randomStrings ORDER BY RAND());
    RETURN @finalStr;
END

I can't do this because:

Msg 443, Level 16, State 1, Procedure getRandomName, Line 6
Invalid use of a side-effecting operator 'rand' within a function.

The postings I have found online related to this problem suggest passing in a random value as a parameter when calling the function, or using a view and querying that view in the function to get a single random number into a variable. I can't use those methods because I am trying to use the randomization in the ORDER BY clause.

Is there a way to accomplish this?

(SQL Server 2014)

---

EDIT:

So you could use a view to get a result as stated below, but now I find myself needing to pass a parameter to the function:

CREATE FUNCTION getRandomName  (
    @maxPieceSize int
)
    RETURNS VARCHAR(48)
AS BEGIN
    -- concatenate two random strings from two columns in a table and return as a new string
    DECLARE @finalStr VARCHAR(48);
    SET @finalStr = (SELECT TOP 1 st1 FROM randomStrings WHERE LEN(st1) <= @maxPieceSize ORDER BY RAND()) + 
        ' ' + 
        (SELECT TOP 1 st2 FROM randomStrings WHERE LEN(st1) <= @maxPieceSize ORDER BY RAND());
    RETURN @finalStr;
END

So I can't create a view for this scenario because you can't pass parameters to views.

So here's my dilemma:

• Function: I can't use this because I cannot use any nondeterministic function within a function.
• View: I can't use this because I need to pass a parameter to the "function".
• Procedure: The only way I can see to do this is to use an output variable, which means declaring a variable, etc. I would not be able to simply do something like EXECUTE getRandomName(6) or SELECT getRandomName(6).

Am I stuck using a procedure and doing it "the hard way" (using an output variable, and having to declare that variable every time I want to use the method)?

---

EDIT AGAIN:

I tried to write the actual method as a stored procedure, then call that stored procedure from a function which declares the variable, assigns it and then returns it. It made sense. Except....

Msg 557, Level 16, State 2, Line 1
Only functions and some extended stored procedures can be executed from within a function.

I'm guessing SQL Server really doesn't want me to have a function that can return a random value. (Funny, because isn't RAND() a function in its own right?)

Answer 1

Why do you stuck with function? Use a view as a function:

CREATE view getRandomName
AS 
    SELECT (SELECT TOP 1 st1 FROM randomStrings ORDER BY Newid()) + 
        ' ' + 
        (SELECT TOP 1 st1 FROM randomStrings ORDER BY Newid())
as RandomName
GO
SELECT (SELECT RandomName FROM getRandomName) + ' - This is random name'
GO

There is also an old and crazy way to get random row within Stored Procedure:

CREATE PROCEDURE usp_Random_Message
@i INT
AS
SELECT TOP 1 * FROM (
    SELECT TOP (@i) * FROM sys.Messages
    ORDER BY message_id
) AS a ORDER BY message_id DESC
GO
DECLARE @i INT = CAST(RAND() * 100 as INT);
EXEC usp_Random_Message @i;

Answer 2

First of all,

SELECT TOP 1 st1 FROM randomStrings ORDER BY RAND()

would not return what you expect, because RAND is a run-time constant. Which means that the server generates a random number once and uses it for the duration of the query.

You want to arrange all rows in a random order and then pick the top row. The following query would do it:

SELECT TOP 1 st1 FROM randomStrings ORDER BY NEWID()

or

SELECT TOP 1 st1 FROM randomStrings ORDER BY CRYPT_GEN_RANDOM(4)

If you look at the execution plan you'll see that the randomStrings table is scanned in full, then sorted and one top row is picked.

I'm guessing that you want to use your function like this:

SELECT
    SomeTable.SomeColumn
    ,dbo.GetRandomName() AS RandomName
FROM SomeTable

For each row in SomeTable you want to get some random string.

Even if you make your original approach work through some tricks, you would have randomStrings table scanned in full and sorted (twice) for each row of the SomeTable. It is likely to be not efficient.

---

One way to make it efficient and avoid tricks is to make sure that the randomStrings table has a int column ID with values from 1 to the maximum number of rows in this table. Make it primary key as well.

Then your function would accept two parameters - two random numbers in the range 1..N and the function would build the random string using the given IDs.

The function may look like this:

CREATE FUNCTION dbo.GetRandomName
(
    @ParamID1 int
    ,@ParamID2 int
)
    RETURNS VARCHAR(48)
AS
BEGIN
    DECLARE @FinalStr VARCHAR(48);
    SET @FinalStr = 
        (SELECT st1 FROM randomStrings WHERE ID = @ParamID1)
        + ' ' +
        (SELECT st1 FROM randomStrings WHERE ID = @ParamID2)
    ;
    RETURN @FinalStr;
END

If randomStrings table has 100 rows with IDs from 1 to 100, then usage of this function may look like this:

SELECT
    SomeTable.SomeColumn
    ,dbo.GetRandomName(
        (CAST(CRYPT_GEN_RANDOM(4) as int) / 4294967295.0 + 0.5) * 100 + 1
        ,(CAST(CRYPT_GEN_RANDOM(4) as int) / 4294967295.0 + 0.5) * 100 + 1
    ) AS RandomName
FROM SomeTable

CRYPT_GEN_RANDOM(4) generate 4 random bytes, they are cast to int and converted to a float number between 0 and 1, which is multiplied by the number of rows in the randomStrings table (100). It is just one of the methods to generate a random number in the range 1...N

CRYPT_GEN_RANDOM generates a different random number each time it is called and it is called twice per row, so you should get expected results.